Facile Synthesis of a Bi 2 WO 6 /BiO 2 − x Heterojunction for Efﬁcient Photocatalytic Degradation of Ciproﬂoxacin under Visible Light Irradiation

: In this work, a Z-scheme Bi 2 WO 6 /BiO 2 − x heterojunction was successfully prepared using a self-assembly strategy. Various characterization techniques demonstrated that the formation of the heterojunction not only accelerated the separation of photoinduced carriers but also weakened the recombination rate of photogenerated electron–hole pairs. The Bi 2 WO 6 /BiO 2 − x composites had a wider absorption edge than Bi 2 WO 6 in the range of 200–800 nm, which improved the photocatalytic performance of ciproﬂoxacin (CIP) degradation under xenon lamps. As a result, the Z-scheme heterojunction Bi 2 WO 6 /BiO 2 − x composite exhibited excellent photocatalytic activity. Cat-alyzed by the optimal 20% Bi 2 WO 6 /BiO 2 − x (0.5 g/L), the removal rate of CIP (10.0 mg/L) was 91.8% within 2 h irradiated by visible light, which was 2.37 times that of the BiO 2 − x catalyst. This work will provide a fresh perspective on the construction of visible-driven Z-scheme photocatalysts for wastewater treatment.


Introduction
Nowadays, antibiotics play an influential role in healthcare, agriculture and animal husbandry [1].For example, ciprofloxacin (CIP), a second-generation quinolone antibacterial drug with broad-spectrum antibacterial and bactericidal activity, is commonly used to treat bacterial infections in humans and animals [2].However, these drugs are not easily broken down in the living body; they remain in feces and eventually accumulate in rivers, pools and groundwater [3].Through wastewater discharges and agricultural runoff, residual drugs can easily enter the aquatic environment, leading to the proliferation of drug-resistant bacteria that can lead to serious ecological risks [4].It is imperative that antibiotics such as CIP should be removed from the ecosystem to ensure public and environmental health.
A surprising finding in recent years is that all inorganic bismuth-based compounds have bright prospects for solar energy conversion and environmental remediation [25][26][27].Among these, BiO 2−x was extensively reported to be a promising candidate for photocatalytic applications due to its relatively narrow band gap and plentiful oxygen vacancies [28].Meanwhile, owing to the existence of mixed Bi 3+ and Bi 5+ , BiO 2−x overcomes the drawbacks of inadequate light absorption range of a monovalent compound [29].Bi 2 WO 6 and BiO 2−x have received a lot of attention in the field of photocatalysis and wastewater treatment field due to their great potential for applications.However, few studies focus on the Bi 2 WO 6 /BiO 2−x -based heterojunction photocatalysis for CIP degradation.At the same time, the mechanism of the photocatalytic reactions in binary bismuth systems with oxygen defects is still unclear.
Here, a series of Z-scheme Bi 2 WO 6 /BiO 2−x heterostructures were fabricated using a facile electrostatic self-assembly process [30].The photocatalytic performance of thecatalysts was verified by measuring its photodegradation performance of CIP.The application of a number of catalysts to the photocatalytic degradation of ciprofloxacin is shown in Table S3, demonstrating that synthesized heterostructures have a prominent effect on the photocatalytic degradation of ciprofloxacin.Electrochemical tests and photoluminescence (PL) spectra proved that the participation of Bi 2 WO 6 could effectively inhibit the recombination of photogenerated carriers in the Bi 2 WO 6 /BiO 2−x heterojunction.In short, constructing Bi 2 WO 6 /BiO 2−x heterojunction restrains the recombination of photogenerated carriers, therefore optimizing broad-spectrum driven photocatalytic activity [31].In addition, based on the photocatalyst characterization and experimental evidence, this work provides a comprehensive understanding of the constructed Z-scheme heterojunction for CIP degradation processes and paves a possible way for the development of extremely efficient photocatalysts for wastewater treatment.

Synthesis and Characterization of Catalysts
The crystalline structures of the parent BiO 2−x , Bi 2 WO 6 and Bi 2 WO 6 /BiO 2−x composites were analyzed through their XRD patterns.As shown in Figure 1a, the characteristic peaks of BiO 2−x were located at 2θ = 28.21, corresponding to the (111), ( 200), (220), (311), and (222) crystal faces, respectively (JCPDS NO. 47-1057) [32], whereas the peaks at 2θ = 28.30[33].As shown in Table S1, we calculated the XRD data using the Scherrer formula, and found that the average grain size of the sample decreased with the increase in Bi 2 WO 6 loading.The decrease of catalyst grain size will increase the specific surface area, which can further increase the contact area with the reactant and thus improve the photocatalytic performance.The increase in FWHM in Bi 2 WO 6 /BiO 2−x is attributed to lattice strain in the heterostructure, which may be due to the doping [17].The Bi 2 WO 6 /BiO 2−x composites displayed similar structural features to BiO 2−x and Bi 2 WO 6 .It reveals that the crystal structure of the composite remains stable during synthesis, further confirming the heterostructure formation of Bi 2 WO 6 /BiO 2−x .
The FT-IR spectra of a series of Bi 2 WO 6 /BiO 2−x composites exhibited similar vibration modes to BiO 2−x , suggesting small amounts of Bi 2 WO 6 combined with BiO 2−x did not affect the chemical structure of BiO 2−x (Figure 1b).In the spectrum of BiO 2−x , the absorption peaks at 519 cm −1 , 589 cm −1 and 954 cm −1 belonged to the stretching vibrations of the Bi-O bond [33].For Bi 2 WO 6 , the characteristic peaks at 589 cm −1 and 687 cm −1 could be ascribed to the stretching vibration of Bi-O and W-O [34].The peak at 1389 cm −1 was associated with the W-O-W bridging stretching of Bi 2 WO 6 [35].There were two signal peaks located at 1635 cm −1 and 3421 cm −1 that belong to the stretching and bending vibrations of the The FT-IR spectra of a series of Bi2WO6/BiO2-x composites exhibited similar vibration modes to BiO2-x, suggesting small amounts of Bi2WO6 combined with BiO2-x did not affect the chemical structure of BiO2-x (Figure 1b).In the spectrum of BiO2-x, the absorption peaks at 519 cm -1 , 589 cm -1 and 954 cm -1 belonged to the stretching vibrations of the Bi-O bond [33].For Bi2WO6, the characteristic peaks at 589 cm -1 and 687 cm -1 could be ascribed to the stretching vibration of Bi-O and W-O [34].The peak at 1389 cm -1 was associated with the W-O-W bridging stretching of Bi2WO6 [35].There were two signal peaks located at 1635 cm -1 and 3421 cm -1 that belong to the stretching and bending vibrations of the O-H functional groups resulting from the adsorption water, respectively [36].All these characteristic peaks of BiO2-x and Bi2WO6 existed in their composites, which proved that Bi2WO6/BiO2x heterojunction had been successfully formed.
Furthermore, Figure 1c shows the Raman spectroscopy of Bi2WO6.For the pure Bi2WO6 sample, the peaks at 158 cm -1 , 305 cm -1 and 825 cm -1 were assigned to the Bi-O, the bending vibrations mode of BiO6 polyhedron and the symmetric stretching vibration of O-W-O [37,38].Figure 1d shows the Raman spectra of BiO2-x and a series of Bi2WO6/BiO2x composites ranging from 100 cm -1 -1200 cm -1 .The peaks at approximately 135 cm -1 , 480 cm -1 and 615 cm -1 could be ascribed to the Bi-O stretches of BiO2-x [39].With the increase in content of Bi2WO6 in the composites, the intensity of the peak at 305 cm -1 also increased, indicating that the Bi2WO6/BiO2-x heterojunction was successfully synthesized.
To acquire the porous characteristics of the obtained materials, we proceeded with N2 adsorption-desorption experiments for BiO2-x, Bi2WO6 and 20% Bi2WO6/BiO2-x. Figure 1e and Table S2 displayed the N2 adsorption-desorption isotherms of the samples; the specific surface area of BiO2-x, Bi2WO6 and 20% Bi2WO6/BiO2-x were 5.1639 m 2 g -1 , 101.5031 m 2 g -1 and 10.4902 m 2 g -1 .According to the IUPAC classification, all of the materials exhibited type IV isotherms featured with type H3 hysteresis loops, suggesting these samples contain abundant mesoporous [40,41].The porous features can be further found in their pore size distribution curves shown in Figure 1f; the pore sizes of Bi2WO6, BiO2-x and 20% Bi2WO6/BiO2-x were dispersed between 8.7 and 53.7 nm.Obviously, the specific surface area of 20% Bi2WO6/BiO2-x was approximately twice that of pure BiO2-x, indicating that the composites had more active sites and were beneficial to enhancing photocatalytic performance.
Figure S1a shows the typical nanosheet morphology of the BiO2-x nanostructure.Bi2WO6 is formed by the accumulation of a large number of nanosheets (Figure S1b).Furthermore, Figure 1c shows the Raman spectroscopy of Bi 2 WO 6 .For the pure Bi 2 WO 6 sample, the peaks at 158 cm −1 , 305 cm −1 and 825 cm −1 were assigned to the Bi-O, the bending vibrations mode of BiO 6 polyhedron and the symmetric stretching vibration of O-W-O [37,38].Figure 1d shows the Raman spectra of BiO 2−x and a series of Bi 2 WO 6 /BiO 2−x composites ranging from 100 cm −1 -1200 cm −1 .The peaks at approximately 135 cm −1 , 480 cm −1 and 615 cm −1 could be ascribed to the Bi-O stretches of BiO 2−x [39].With the increase in content of Bi 2 WO 6 in the composites, the intensity of the peak at 305 cm −1 also increased, indicating that the Bi 2 WO 6 /BiO 2−x heterojunction was successfully synthesized.
To acquire the porous characteristics of the obtained materials, we proceeded with N 2 adsorption-desorption experiments for BiO 2−x , Bi 2 WO 6 and 20% Bi 2 WO 6 /BiO 2−x .Figure 1e and Table S2 displayed the N 2 adsorption-desorption isotherms of the samples; the specific surface area of BiO 2−x , Bi 2 WO 6 and 20% Bi 2 WO 6 /BiO 2−x were 5.1639 m 2 g −1 , 101.5031 m 2 g −1 and 10.4902 m 2 g −1 .According to the IUPAC classification, all of the materials exhibited type IV isotherms featured with type H3 hysteresis loops, suggesting these samples contain abundant mesoporous [40,41].The porous features can be further found in their pore size distribution curves shown in Figure 1f; the pore sizes of Bi 2 WO 6 , BiO 2−x and 20% Bi 2 WO 6 /BiO 2−x were dispersed between 8.7 and 53.7 nm.Obviously, the specific surface area of 20% Bi 2 WO 6 /BiO 2−x was approximately twice that of pure BiO 2−x , indicating that the composites had more active sites and were beneficial to enhancing photocatalytic performance.
Figure S1a shows the typical nanosheet morphology of the BiO 2−x nanostructure.Bi 2 WO 6 is formed by the accumulation of a large number of nanosheets (Figure S1b). Figure S1c and Figure 2a show the SEM and TEM images of 20% Bi 2 WO 6 /BiO 2−x , which prove that Bi 2 WO 6 nanosheets are loaded on BiO 2−x nanosheets successfully.The HRTEM images (Figures 2b and S2b Figure S1c and Figure 2a show the SEM and TEM images of 20% Bi2WO6/BiO2-x, which prove that Bi2WO6 nanosheets are loaded on BiO2-x nanosheets successfully.The HRTEM images (Figure 2b and Figure S2b) show the lattice stripes of 0.315 nm, corresponding to the (111) crystal plane of BiO2-x nanosheets.Moreover, Figure 2b and Figure S2d show that the calculated d space values 0.375 nm and 0.226 nm are attributed to the (111) plane and (042) plane of Bi2WO6.EDS elemental analysis (Figure S3), and the elemental mapping images (Figure 2d-f) of 20% Bi2WO6/BiO2-x reveal homogeneous distribution of Bi, O, and W elements in the sample, which is highly in accordance with the results of TEM images and indicates the formation of Bi2WO6/BiO2-x heterojunction.X-ray photoelectron spectroscopy (XPS) technology was performed to elucidate the surface element composition and valence state of the prepared catalysts and the interaction between BiO2-x and Bi2WO6.As shown in Figure 3a, in addition to the characteristic peaks of Bi and O, there are also peaks of W in the 20% Bi2WO6/BiO2-x heterostructure.Figure 3b-d shows the XPS high-resolution spectra of O, Bi and W XPS, respectively.For Bi2WO6/BiO2-x hybrid, the deconvolution of the high-resolution spectrum of O 1s (Figure 3b) gives three peaks at 529.5 eV, 530.4 eV and 531.8 eV, assignable to lattice oxygen [42], surface hydroxyl group [37] and oxygen vacancies [43] in the as-prepared samples, respectively.The Bi 4f7/2 and 4f5/2 peaks are situated at 158.3 eV and 163.6 eV, 159.1 eV and 164.5 eV for BiO2-x and Bi2WO6, respectively [39].Furthermore, the peak positions of Bi 4f for 20% Bi2WO6/BiO2-x were positively shifted relative to BiO2-x, which were negatively shifted relative to Bi2WO6 (Figure 3c).The W 4f7/2 and W 4f5/2 peaks of 20% Bi2WO6/BiO2-x are centered at 35 eV and 37.2 eV [44], which are 0.4 eV lower than those of the Bi2WO6 sample (35.4 eV of W 4f7/2 and 37.6 eV of W 4f5/2) (Figure 3d).This phenomenon reveals the successful synthesis of Bi2WO6/BiO2-x heterojunction.X-ray photoelectron spectroscopy (XPS) technology was performed to elucidate the surface element composition and valence state of the prepared catalysts and the interaction between BiO 2−x and Bi 2 WO 6 .As shown in Figure 3a, in addition to the characteristic peaks of Bi and O, there are also peaks of W in the 20% Bi 2 WO 6 /BiO 2−x heterostructure.Figure 3b-d shows the XPS high-resolution spectra of O, Bi and W XPS, respectively.For Bi 2 WO 6 /BiO 2−x hybrid, the deconvolution of the high-resolution spectrum of O 1s (Figure 3b) gives three peaks at 529.5 eV, 530.4 eV and 531.8 eV, assignable to lattice oxygen [42], surface hydroxyl group [37] and oxygen vacancies [43] in the as-prepared samples, respectively.The Bi 4f 7/2 and 4f 5/2 peaks are situated at 158.3 eV and 163.6 eV, 159.1 eV and 164.5 eV for BiO 2−x and Bi 2 WO 6 , respectively [39].Furthermore, the peak positions of Bi 4f for 20% Bi 2 WO 6 /BiO 2−x were positively shifted relative to BiO 2−x , which were negatively shifted relative to Bi 2 WO 6 (Figure 3c).The W 4f 7/2 and W 4f 5/2 peaks of 20% Bi 2 WO 6 /BiO 2−x are centered at 35 eV and 37.2 eV [44], which are 0.4 eV lower than those of the Bi 2 WO 6 sample (35.4 eV of W 4f 7/2 and 37.6 eV of W 4f 5/2 ) (Figure 3d).This phenomenon reveals the successful synthesis of Bi 2 WO 6 /BiO 2−x heterojunction.

Optical and Photoelectrochemical Property Analysis
Figure 4a displays the UV-Vis DRS spectra of the prepared photocatalysts.The absorption edge of 20% Bi2WO6/BiO2-x displayed an obvious red shift compared with that of Bi2WO6, suggesting that composites could make better use of visible light.The light ab-   4c, such band combinations of Bi 2 WO 6 and BiO 2−x were conducive to the formation of the heterojunction.Furthermore, the photoelectrochemical measurement was also evaluated to investigate the charge separation and transfer efficiency of samples.From Figure 4d, with light switching on and off, the photocurrent densities of the 20% Bi 2 WO 6 /BiO 2−x composite demonstrated the highest photocurrent density over other samples, and also presented the lowest resistance ability, indicating enhanced separation of photogenerated carriers in the composites.According to the electrochemical impedance spectroscopy (EIS) Nyquist plots (Figure 4e), compared with pure Bi 2 WO 6 and BiO 2−x , the formation of Bi 2 WO 6 /BiO 2−x heterojunction could significantly reduce the semi-arc, indicating that the carrier recombination rate of the composite was lowest.In order to further detect the photogenerated electrons and holes recombination, PL emission spectra is commonly employed.As depicted in Figure 4f, the PL intensity of Bi 2 WO 6 significantly decreased when it hybridized with BiO 2−x , indicating that the 20% Bi 2 WO 6 /BiO 2−x samples obtained the superior charge separation ability.In a word, the separation efficiency of photogenerated carriers in the 20% Bi 2 WO 6 /BiO 2−x samples could be greatly enhanced, which could significantly contribute to improving its photocatalytic activity.

Photocatalytic Performance of Catalysts
The photocatalytic performances of BiO2-x, Bi2WO6 and a series of Bi2WO6/BiO2-x composites were explored through CIP degradation under visible light illumination.As indicated in Figure S5, no remarkable CIP decomposition was found without any catalyst under the illumination of visible light, which indicates that the presence of catalysts is necessary for the degradation of CIP.-As Figure 5a,b showed, the degradation percentage of 91.8% was achieved by 20% Bi2WO6/BiO2-x after 120 min of degradation process and its corresponding apparent kinetic constant was 0.02240 min -1 , which was higher than that of

Photocatalytic Performance of Catalysts
The photocatalytic performances of BiO 2−x , Bi 2 WO 6 and a series of Bi 2 WO 6 /BiO 2−x composites were explored through CIP degradation under visible light illumination.As indicated in Figure S5, no remarkable CIP decomposition was found without any catalyst under the illumination of visible light, which indicates that the presence of catalysts is necessary for the degradation of CIP.As Figure 5a,b showed, the degradation percentage of 91.8% was achieved by 20% Bi 2 WO 6 /BiO 2−x after 120 min of degradation process and its corresponding apparent kinetic constant was 0.02240 min −1 , which was higher than that of BiO 2−x by 2.37 times.Thus, the synergistic effect can be achieved using the combination of Bi 2 WO 6 and BiO 2−x via ultrasonication, which may be ascribed to the fact that the formation of Bi 2 WO 6 /BiO 2−x heterojunctions can enhance visible light absorption efficiency, accelerate the transfer rate of photo-generated electron-hole pairs and reinforce the oxidation ability for CIP decomposition.

Photocatalytic Performance of Catalysts
The photocatalytic performances of BiO2-x, Bi2WO6 and a series of Bi2WO6/BiO2-x composites were explored through CIP degradation under visible light illumination.As indicated in Figure S5, no remarkable CIP decomposition was found without any catalyst under the illumination of visible light, which indicates that the presence of catalysts is necessary for the degradation of CIP.-As Figure 5a,b showed, the degradation percentage of 91.8% was achieved by 20% Bi2WO6/BiO2-x after 120 min of degradation process and its corresponding apparent kinetic constant was 0.02240 min -1 , which was higher than that of BiO2-x by 2.37 times.Thus, the synergistic effect can be achieved using the combination of Bi2WO6 and BiO2-x via ultrasonication, which may be ascribed to the fact that the formation of Bi2WO6/BiO2-x heterojunctions can enhance visible light absorption efficiency, accelerate the transfer rate of photo-generated electron-hole pairs and reinforce the oxidation ability for CIP decomposition.To evaluate the stability of 20% Bi 2 WO 6 /BiO 2−x , cyclic experiments for the photodegradation of CIP were performed in Figure 5c.20% Bi 2 WO 6 /BiO 2−x could retain its activity well for four catalytic cycles, indicating the great stability of 20% Bi 2 WO 6 /BiO 2−x .Moreover, the SEM image of the used 20% Bi 2 WO 6 /BiO 2−x was almost the same as the fresh one (Figure S1c,d).Furthermore, in the XRD pattern before and after the photocatalytic degradation of CIP of 20% Bi 2 WO 6 /BiO 2−x , it was found that the position of the characteristic peak did not shift, further demonstrating the structural stability during the photocatalysis process (Figure S7).The FTIR of CIP (Figure S6a) showed peaks corresponding to the asymmetric -CH 3 stretching vibrations at 2924.06 cm −1 , ring vibrations at 1036.7 cm −1 , amidogen ν N-H vibrations at 3600 to 2700 cm −1 and the -C-N band finger prints of the azo nature of dye at 1119.1 cm −1 [45].Most importantly, these characteristic peaks of CIP almost disappeared in the presence of 20% Bi 2 WO 6 /BiO 2−x (Figure S6b-d), suggesting the degradation of CIP.These results indicated that the as-prepared 20% Bi 2 WO 6 /BiO 2−x maintained stable photocatalytic activity during long time photo-degradation reactions.
As shown in Figure 5d, the influence of-pH on the photo-degradation of CIP in suspensions of 20% Bi 2 WO 6 /BiO 2−x was investigated in the pH range of 3.0-11.0.The dosage of 20% Bi 2 WO 6 /BiO 2−x and CIP concentration were 0.5 g/L and 10 mg/L, respectively.It can be seen that when the alkalinity of the solution was increased, the degradation efficiency of CIP was inhibited.Although the CIP removal decreased at pH 11.0, about 53.6% of CIP was still removed, proving that 20% Bi 2 WO 6 /BiO 2−x system could be applied over a wide pH range.According to Figure 5e, Na + , K + , Ca 2+ and Mg 2+ had little effect on CIP removal.
In addition, the inorganic anion NO 3 − had little effect on the degradation of CIP, while the CO 3 2− had a significant inhibitory effect on CIP degradation (Figure 5f).Conclusively, the 20% Bi 2 WO 6 /BiO 2−x system exhibited excellent photo-degradation of organic pollutants.

Possible Degradation Pathways
In order to explore the photocatalytic degradation mechanism and possible degradation pathways of CIP, the intermediate products in the degradation of CIP were estimated using HPLC-MS.There were two possible photocatalytic degradation pathways of CIP, as shown in Figure 6

Mechanism Discussion
In order to clarify the main reactive radical species on the degradation of CIP, we carried out scavenging experiments using tertiary butanol (TBA), Na2C2O4, p-benzoquinone (PBQ) and furfural alcohol (FFA) for quenching hydroxyl radicals (•OH), photogenerated holes (h + ), superoxide radicals (•O2 -) and singlet oxygen ( 1 O2), respectively [48].As Figure 7a, b shows, the decomposition efficiency of CIP almost remained when TBA was added to the suspension, implying that less •OH was produced during CIP degradation.In contrast, a sharp decrease in decomposition efficiency was observed when Na2C2O4, PBQ and FFA were added in system; the degradation efficiency of CIP decreased to 39.4%, 45.7% and 42.5%, and the corresponding reaction rate constant decreased to 0.00557 min -1 and 0.00580 min -1 and 0.00590 min -1 .These results indicated that h + played a major role in CIP degradation, •O2 -and 1 O2 were the moderate reactive species in the degradation of CIP over the 20% Bi2WO6/BiO2-x composite.To verify this conclusion, we conducted ESR experiments in Figure 7c-f

Mechanism Discussion
In order to clarify the main reactive radical species on the degradation of CIP, we carried out scavenging experiments using tertiary butanol (TBA), Na 2 C 2 O 4 , p-benzoquinone (PBQ) and furfural alcohol (FFA) for quenching hydroxyl radicals (•OH), photo-generated holes (h + ), superoxide radicals (•O 2 − ) and singlet oxygen ( 1 O 2 ), respectively [48].As Figure 7a,b shows, the decomposition efficiency of CIP almost remained when TBA was added to the suspension, implying that less •OH was produced during CIP degradation.In contrast, a sharp decrease in decomposition efficiency was observed when Na 2 C 2 O 4 , PBQ and FFA were added in system; the degradation efficiency of CIP decreased to 39.4%, 45.7% and 42.5%, and the corresponding reaction rate constant decreased to 0.00557 min −1 and 0.00580 min −1 and 0.00590 min −1 .These results indicated that h + played a major role in CIP degradation, •O 2 − and 1 O 2 were the moderate reactive species in the degradation of CIP over the 20% Bi 2 WO 6 /BiO 2−x composite.To verify this conclusion, we conducted ESR experiments in Figure 7c-f.Upon visible light illumination, the corresponding characteristic peaks intensity of DMPO-•O 2 − , TEMP-1 O 2 and DMPO-•OH in the ESR spectra slightly increased, while the characteristic peaks' intensity of TEMPO-h + decreased significantly after illumination.As illustrated in Figure 7e, the three peaks' intensity of TEMPO-h + would get weaker, demonstrating the existence of photogenerated h + [6].Conclusively, the existence of •OH and h + in CIP photodegradation process was confirmed.
Catalysts 2023, 13, x FOR PEER REVIEW 9 of 14 According to the aforementioned results, the possible CIP photodegradation mechanism was presented in Scheme 1.Under visible light irradiation, both Bi2WO6 and BiO2-x could be photoexcited and produced electron-hole pairs, and the photo-induced electrons migrated from the valence band to the conduction band.Since the ECB of Bi2WO6 was more -positive than the O2/•O2 -potential (-0.33 V vs NHE), •O2 -could not be generated in the CB.Therefore, the type II heterojunction did not conform to the structure of this Bi2WO6/BiO2-x composites, that a Z-scheme charge transfer way was proposed for this degradation process.The photogenerated electrons in the CB of Bi2WO6 could migrate to the VB of BiO2-x and combine with h + by visible light illuminating.In the VB of Bi2WO6 and the CB of BiO2-x, •OH and •O2 -could be produced, respectively.Moreover, most of the •O2 -further reacted to generate 1 O2.These active species gradually converted CIP into small molecules that were easily broken down.Combined with ESR and radical quenching reactions, the overall electron transition and CIP degradation reaction in Bi2WO6/BiO2x heterostructures were proposed as follows (Equations ( 1)-( 7)) [49][50][51]: O2 + e -→ •O2 - H2O ↔ H + + OH - According to the aforementioned results, the possible CIP photodegradation mechanism was presented in Scheme 1.Under visible light irradiation, both Bi 2 WO 6 and BiO 2−x could be photoexcited and produced electron-hole pairs, and the photo-induced electrons migrated from the valence band to the conduction band.Since the E CB of Bi 2 WO 6 was more -positive than the O 2 /•O 2 − potential (−0.33 V vs. NHE), •O 2 − could not be generated in the CB.Therefore, the type II heterojunction did not conform to the structure of this Bi 2 WO 6 /BiO 2−x composites, that a Z-scheme charge transfer way was proposed for this degradation process.The photogenerated electrons in the CB of Bi 2 WO 6 could migrate to the VB of BiO 2−x and combine with h + by visible light illuminating.In the VB of Bi 2 WO 6 and the CB of BiO 2−x , •OH and •O 2 − could be produced, respectively.Moreover, most of the •O 2 − further reacted to generate 1 O 2 .These active species gradually converted CIP into small molecules that were easily broken down.Combined with ESR and radical quenching reactions, the overall electron transition and CIP degradation reaction in Bi 2 WO 6 /BiO 2−x heterostructures were proposed as follows (Equations ( 1)-( 7)) [49][50][51]: Catalysts 2023, 13, x FOR PEER REVIEW 10 o Scheme 1.The proposed photocatalytic mechanism for photodegradation of CIP over Bi2WO6/BiO2-x composites.

Preparation of BiO2-x
BiO2-x was prepared using a hydrothermal method.Briefly, 1.2 g NaOH and 1 NaBiO3⋅2H2O were incorporated into 30 mL of ultrapure water via magnetic stirring.A stirring for 30 min, the suspension was transferred into a 50 mL autoclave, and heate 180 °C for 18 h.The mixture was naturally cooled to room temperature and collected centrifugation, washed with ultrapure water and dried at 80 °C for 4 h under vacuum

Preparation of Bi2WO6
Firstly, 0.3881g Bi(NO3)3⋅5H2O and 0.1319 g Na2WO4⋅2H2O were mixed together i mL of ethylene glycol and stirred for 30 min to obtain the homogeneous solution.It transferred into a 100 ml Teflon-lined autoclave and treated at 160 °C for 15 h.After c ing at 25 °C, the product was centrifuged and rinsed several times with ethanol and trapure water to remove residual inorganic ions.Finally, Bi2WO6 powders were ov dried at 70 °C for 12 h.

Preparation of Bi2WO6/BiO2-x
The Bi2WO6/BiO2-x composite was prepared via an electrostatic self-assembly met illustrated in Scheme 2. A total of 0.10 g of BiO2-x and different amounts of Bi2WO6 (0. 0.01, 0.015, 0.02 and 0.025 g) were added to 30 mL ethanol with ultrasonics for 4 h, dried at 60 °C for 4 h under vacuum.

Preparation of BiO 2−x
BiO 2−x was prepared using a hydrothermal method.Briefly, 1.2 g NaOH and 1.4 g NaBiO 3 •2H 2 O were incorporated into 30 mL of ultrapure water via magnetic stirring.After stirring for 30 min, the suspension was transferred into a 50 mL autoclave, and heated at 180 • C for 18 h.The mixture was naturally cooled to room temperature and collected via centrifugation, washed with ultrapure water and dried at 80 • C for 4 h under vacuum.

Preparation of Bi 2 WO 6
Firstly, 0.3881 g Bi(NO 3 ) 3 •5H 2 O and 0.1319 g Na 2 WO 4 •2H 2 O were mixed together in 40 mL of ethylene glycol and stirred for 30 min to obtain the homogeneous solution.It was transferred into a 100 mL Teflon-lined autoclave and treated at 160 • C for 15 h.After cooling at 25 • C, the product was centrifuged and rinsed several times with ethanol and ultrapure water to remove residual inorganic ions.Finally, Bi 2 WO 6 powders were oven-dried at 70 • C for 12 h.

Preparation of Bi 2 WO 6 /BiO 2−x
The Bi 2 WO 6 /BiO 2−x composite was prepared via an electrostatic self-assembly method illustrated in Scheme 2. A total of 0.10 g of BiO 2−x and different amounts of Bi 2 WO 6 (0.005, 0.01, 0.015, 0.02 and 0.025 g) were added to 30 mL ethanol with ultrasonics for 4 h, and dried at 60 • C for 4 h under vacuum.

Conclusions
In summary, we have successfully prepared a Z-scheme Bi2WO6/BiO2-x heterostructures using a simple method with excellent photocatalytic CIP degradation performance under visible light irradiation.By constructing a Z-scheme heterostructure, the charge recombination rate was significantly reduced in the photocatalytic degradation of ciprofloxacin.The Z-scheme charge transfer mechanism in Bi2WO6/BiO2-x heterojunction as investigated via XPS, photoelectrochemical measurements and ESR experiment greatly enhances the separation of photogenerated carriers to expedite CIP photodegradation.With the increasing Bi2WO6 content, the average grain size of the samples decreases, and the surface area of the materials in contact with the pollutant increases, which improves the photocatalytic performance.The 20% Bi2WO6/BiO2-x samples were constructed to alleviate the problem of the high recombination rate of photogenerated e --h + , and the CIP degradation rate reaches 91.8% under the present conditions.The 20% Bi2WO6/BiO2-x heterostructures still had good stability and photocatalytic activity after four cycles, showing high potential in practical applications.Critically, this study demonstrates the successful application of CIP degradation and provides valuable insights regarding the development of the construction of heterojunctions to further improve their photocatalytic degradation efficiency.

Conclusions
In summary, we have successfully prepared a Z-scheme Bi 2 WO 6 /BiO 2−x heterostructures using a simple method with excellent photocatalytic CIP degradation performance under visible light irradiation.By constructing a Z-scheme heterostructure, the charge recombination rate was significantly reduced in the photocatalytic degradation of ciprofloxacin.The Z-scheme charge transfer mechanism in Bi 2 WO 6 /BiO 2−x heterojunction as investigated via XPS, photoelectrochemical measurements and ESR experiment greatly enhances the separation of photogenerated carriers to expedite CIP photodegradation.With the increasing Bi 2 WO 6 content, the average grain size of the samples decreases, and the surface area of the materials in contact with the pollutant increases, which improves the photocatalytic performance.The 20% Bi 2 WO 6 /BiO 2−x samples were constructed to alleviate the problem of the high recombination rate of photogenerated e − -h + , and the CIP degradation rate reaches 91.8% under the present conditions.The 20% Bi 2 WO 6 /BiO 2−x heterostructures still had good stability and photocatalytic activity after four cycles, showing high potential in practical applications.Critically, this study demonstrates the successful application of CIP degradation and provides valuable insights regarding the development of the construction of heterojunctions to further improve their photocatalytic degradation efficiency.
) show the lattice stripes of 0.315 nm, corresponding to the (111) crystal plane of BiO 2−x nanosheets.Moreover, Figures2b and S2dshow that the calculated d space values 0.375 nm and 0.226 nm are attributed to the (111) plane and (042) plane of Bi 2 WO 6 .EDS elemental analysis (FigureS3), and the elemental mapping images (Figure2d-f) of 20% Bi 2 WO 6 /BiO 2−x reveal homogeneous distribution of Bi, O, and W elements in the sample, which is highly in accordance with the results of TEM images and indicates the formation of Bi 2 WO 6 /BiO 2−x heterojunction.Catalysts 2023, 13, x FOR PEER REVIEW 4 of 14

Figure
Figure4adisplays the UV-Vis DRS spectra of the prepared photocatalysts.The absorption edge of 20% Bi 2 WO 6 /BiO 2−x displayed an obvious red shift compared with that of Bi 2 WO 6 , suggesting that composites could make better use of visible light.The light absorption edge of Bi 2 WO 6 had a band gap of 2.54 eV, while BiO 2−x had a narrower band gap of 1.34 eV (Figure4b).The positive slope of the Mott-Schottky curves in FigureS4a,b indicates that both Bi 2 WO 6 and BiO 2−x are n-type semiconductors and the flat band positions of Bi 2 WO 6 and BiO 2−x are −0.289 and −0.436 V (relative to Ag/AgCl).Furthermore, according to the formulas of E NHE = E (Ag/AgCl) + 0.197 and E CB = E flat band − 0.1 (n-type semiconductor), the E CB values of Bi 2 WO 6 and BiO 2−x are calculated to −0.192 and −0.339V vs. NHE, respectively.Additionally, based on the formula E VB = E CB + E g , the valence band positions (E VB ) of Bi 2 WO 6 and BiO 2−x are calculated to −2.348 and −1.001eV vs. NHE, respectively.As shown in Figure4c, such band combinations of Bi 2 WO 6 and BiO 2−x were conducive to the formation of the heterojunction.Furthermore, the photoelectrochemical measurement was also evaluated to investigate the charge separation and transfer efficiency of samples.From Figure4d, with light switching on and off, the photocurrent densities of the 20% Bi 2 WO 6 /BiO 2−x composite demonstrated the highest photocurrent density over other samples, and also presented the lowest resistance ability, indicating enhanced separation of photogenerated carriers in the composites.According to the electrochemical impedance spectroscopy (EIS) Nyquist plots (Figure4e), compared with pure Bi 2 WO 6 and BiO 2−x , the formation of Bi 2 WO 6 /BiO 2−x heterojunction could significantly reduce the semi-arc, indicating that the carrier recombination rate of the composite was lowest.In order to further detect the photogenerated electrons and holes recombination, PL emission spectra is commonly employed.As depicted in Figure4f, the PL intensity of Bi 2 WO 6 significantly decreased when it hybridized with BiO 2−x , indicating that the 20% Bi 2 WO 6 /BiO 2−x samples obtained the superior charge separation ability.In a word, the separation efficiency of photogenerated carriers in the 20% Bi 2 WO 6 /BiO 2−x samples could be greatly enhanced, which could significantly contribute to improving its photocatalytic activity.

Figure 5 .Figure 5 .
Figure 5. (a) Photocatalytic degradation of CIP using various photocatalysts irradiated with visible light and (b) their corresponding kinetic curves, (c) The cyclic experiment of the 20% Bi2WO6/BiO2-x system for degradation of CIP.The effects of (d) initial solution pH, (e) inorganic cations and (f) Figure 5. (a) Photocatalytic degradation of CIP using various photocatalysts irradiated with visible light and (b) their corresponding kinetic curves, (c) The cyclic experiment of the 20% Bi 2 WO 6 /BiO 2−x system for degradation of CIP.The effects of (d) initial solution pH, (e) inorganic cations and (f) anions in 20% Bi 2 WO 6 /BiO 2−x system for degradation of CIP under visible light irradiation.(Condition: sample dosage = 0.5 g/L, CIP concentration: 10 mg/L).

Scheme 1 .
Scheme 1.The proposed photocatalytic mechanism for photodegradation of CIP over the Bi 2 WO 6 /BiO 2−x composites.